Coating compositions for metal containers require physical properties that withstand the processing conditions in manufacturing the metal containers. Lids for metal containers, for example, are typically manufactured by first coating flat sheets of a metal substrate, heating the coated substrate and then stamping or shaping the coated substrate into a container having a desired shape.
Coating compositions generally contain a film-forming synthetic resin component which is applied to metal sheet substrate in the form of dispersion and is then heat-cured to a dried film. The coatings applied to food and beverage can ends, for example, are applied in film weights of approximately 1 milligram per square inch to approximately 9 milligrams per square inch on high speed coil coating lines. High speed coating lines require a coating material that will dry and cure within a few seconds as it is heated very rapidly to peak metal temperatures that can range from approximately 200° C. to approximately 300° C. (392° F. to about 550° F.).
Polyvinylchloride (PVC)-based coatings have been used to coat the interior and exterior of metal containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion to a metal substrate, flexibility, chemical resistance, and corrosion inhibition. These PVC containing coatings, however, are being replaced by solvent and water-borne epoxy systems in order to eliminate excessive levels of bisphenol A diglycidyl ether, a stabilizer used in PVC coatings, and to eliminate the potential of dioxin production if the PVC coating is improperly burned. Concerned industrial applicators are acting to eliminate these polyvinylchloride-based coating compositions to eliminate the environmental and health concerns associated with them.
Due to these concerns and the environmental concerns associated with organic solvents, there is an increasing demand for aqueous coating compositions. Water-borne epoxy systems have been used to coat the outside of easy open ends of beverage cans, for example, but their use has caused problems in productivity and quality on certain tooling designs.
More specifically, repeated stamping of a coil that is coated with alternative coating compositions, i.e. those which do not contain PVC, can result in build up of the coating on the forming tool as well as a scuffed coating on the can ends. When coated metal coil is fed into a forming press, the coating is contacted by the tool under pressure as the tool forms the shape of the can end, and then the tool is separated from the surface of the coated metal. The repeated contact of the tool with the coating during shaping produces a gradual build-up of the coating on the tool. Also, the coating along the perimeter of the can end is scuffed as the formed can ends are separated from the tool. This scuffing problem not only results in can ends having thickened edges, but also results in the need for high maintenance efforts surrounding the forming press and, in some cases, results in complete shutdown of a production line.